Noise from Matchless DC30 EF86 Channel

[nee]

Got a bad case of "roaring surf" from channel 2 (the EF86) channel of my Matchless DC30!

Symptoms include:

1) Volume pot is noisy when it is turned. "Surf" noise increases/decreases with volume.

2) Six-way tone switch makes loud pops when switched. Quality of noise changes in line with tone position, i.e it's bassy in position 1 and gets progressively thinner from 2 thru 6.

I've tried other tubes, wiggled the tube etc.

[fishy]

Not looked at a DC30 before but it sounds like a leaky coupling cap to me. Probably have DC on the switch and pot causing popping and the scratchy noise.

[nee]

Thanks fishy. Not that I know too much about it, but DC is the thought that crossed my mind.

In the schematic, the signal goes straight from the EF86 to the 6 caps for the tone switch, and then straight to the volume control, so (I guess) the tone caps are the coupling caps?? The only other caps before the tone caps are the usual cathode bypass and grid caps on the (pentode) tube.

[fishy]

The caps off the plate are the coupling / tone caps.
Having not seen one I'm not the right person to advise on the best approach from here though.
Thinking out loud.......maybe see if you have DC on the input to the pot and see if you get a higher voltage on one particular switch setting to highlight a cap / caps.

Might be wrong on this though. Sounds a bit too easy.

[ant]

I had an ef86 channel in my 18 watt but got rid of it cause the tube went noisy as you describe.

[dartanion]

Good EF86s are hard to find, so buy a bunch of them as cheap as you can get them and weed through them, Slightly microphonic tubes are useable in some amps, others require dead stable tubes only. There are some known solid subs for EF86s out there.

Try a different tube first for the surfing noise.

Post some gut shots so we can help you troubleshoot better.

[nee]

Thanks all. It was indeed one of those tone caps that went bad.

There are six tone caps varying from .01uf down to 360pf. The .01uf cap has a 180pf cap in parallel, and it was this little one that was bad. Not quite sure how much of a difference that 180pf would make?? .01uf plus 180pf = .01018uf. Not much of a change I would have thought.

Oddly enough I've had no trouble finding tubes. Got an RFI in there now, but have tried a NOS Tesa and an EH with no problem. True though - if you've got a noisy or microphonic one you'll know it straight away.

[dartanion]

The paralleled caps is a mojo thing. Try different cap types and variations here for tonal flavoring.

[Mastervolume]

also remove that .1 cathode cap off of v1. You will find that OD pedals will work better w/o it.

[nee]

also remove that .1 cathode cap off of v1. You will find that OD pedals will work better w/o it.

I'd wondered about that cap. What does it actually do? Excuse my newbidity!

[jelle]

also remove that .1 cathode cap off of v1. You will find that OD pedals will work better w/o it.

If you do that make sure you have at least a 47pf cathode bypass. EF86's are no ECC83's! The Datasheet calls for it for stability reasons.

jelle